To purify the exhaust gas in the case of internal combustion engines which operate on the Otto principle, a three-way catalytic converter is usually located in the exhaust gas tract of the internal combustion engine. Upstream from this catalytic converter is a lambda probe, which delivers a signal that is a function of the proportion of residual oxygen contained in the exhaust gas. This residual oxygen content is in turn dependent on the mixture which is fed into the internal combustion engine. When there is an excess of fuel (rich mixture, or air volumes with lambda<1), the proportion of oxygen in the raw exhaust gas is lower, and when there is an excess of air during combustion (lean mixture or air volumes with lambda>1) the proportion is higher.
The lambda probes which are usually used upstream from the catalytic converter, which because of their position are also referred to as pre-converter lambda probes, are so-called binary or step probes. For these, when the mixture is lean (lambda>1), the output voltage usually lies below 100 mV. For stochiometric combustion with lambda=1, the output voltage increases in a step-like fashion. With a rich mixture (lambda<1), the output voltage reaches values over 0.6 V; this is described as two-point behavior. It is characteristic of this two-point behavior of binary lambda probes that, in the region where the characteristic curve exhibits a steep slope, the signal delivered by the lambda probe is therefore very strongly dependent on the value of lambda for the exhaust gas. As the mixture becomes richer from a point close to a lambda value of 1, the slope of the characteristic curve flattens off significantly. With currently-available binary lambda probes, the kink in the characteristic curve which this produces lies at around lambda=0.998.
There are also lambda probes which supply a unique, strictly monotonically increasing signal over a wide range of lambda values (between about 0.7 and 4). These lambda probes are referred to as linear lambda probes or broadband lambda probes.
An internal combustion engine running under closed loop lambda control operates in such a way that the output signal from the lambda probe, which reflects the lambda value for the raw exhaust gas, fluctuates about a predetermined mean value, which corresponds roughly to lambda=1. Because a three-way catalytic converter exhibits its optimal catalytic properties for a raw exhaust gas with a certain value λ0 of lambda, the predetermined mean value should also actually correspond to λ0. Depending on the catalytic converter, the value λ0 of lambda for which the optimal catalytic effect is achieved, can lie at a value which differs slightly from lambda=1, for example at lambda=0.99, or in particular lambda=0.998.
The dynamic and static characteristics of each lambda probe change as a result of aging and contamination. This causes the position of the signal level which corresponds to λ0 to be displaced. The familiar way of dealing with this problem is to locate a further lambda probe downstream from the three-way catalytic converter so that, because of its lower thermal loading and its position downstream from the catalytic converter, it is subject to less serious attacks from chemically aggressive substances. This lambda probe, which because of its position downstream from the catalytic converter is also referred to as a post-converter lambda probe, serves as a monitoring probe, for monitoring the catalytic conversion, and permits fine regulation of the fuel/air mixture, in that the signal level from the pre-converter lambda probe which is set to correspond to λ0 is corrected so that the value λ0 of lambda which is most the favorable for the catalytic conversion can always be adhered to on average. This method is described as closed loop guidance or trimming control.
DE 198 19 461 A1 describes a closed loop trimming method with which the signal from an NOx-sensitive transducer, located downstream from a three-way catalytic converter, is used instead of the signal from a post-converter lambda probe. A similar closed loop trimming method which uses an NOx-sensitive transducer is described in DE 198 52 244 C1.
As progress has been made in the reduction of the pollutants emitted by an internal combustion engine, three-way catalytic converters have become available which exhibit a significantly increased conversion rate for hydrocarbons, carbon monoxide and oxides of nitrogen. However, it has been found that such high-efficiency catalytic converters change the behavior of the post-converter lambda probes so that in effect the characteristic curve for the probe in the rich fuel/air mix region, i.e. for lambda values<1, has a significantly flatter slope than for factory-fresh probes or for aged probes which have been operated with conventional three-way catalytic converters. Furthermore, aging also generally leads to a displacement of the signal level, i.e. to a change in the offset, so that in the region of rich fuel/air mixtures the signal level decreases, which means that it is no longer possible to evaluate the signal reliably because it lies outside the manufacturer's specifications. This displacement by an offset also heightens the problem of the flattening of the curve. With probes which have aged in this way, it is no longer possible to exercise trimming control with the necessary accuracy, or the desired longevity of the post-converter lambda probe is not achieved.